# 12 Analyzing Classful IPv4 Networks
## Classful Network Concepts
■ Class (A, B, or C)
■ Default mask
■ Number of network octets/bits
■ Number of host octets/bits
■ Number of host addresses in the network
■ Network ID
■ Network broadcast address
■ First and last usable address in the network

### IPv4 Network Classes and Related Facts

Note that the address ranges of **all addresses that begin with 0 and all addresses that begin with 127 are reserved.**

### The Number and Size of the Class A, B, and C Networks

#### Address Formats

- The addresses in the same network have the same values in the network part.
- The addresses in the same network have different values in the host part.

#### Default Masks
For example, Class A network 10.0.0.0 has a network part of the first single octet (8 bits) and a host part of the last three octets (24 bits). **As a result, the Class A default mask is 255.0.0.0, which in binary is 11111111 00000000 00000000 00000000**


#### Number of Hosts per Network
**Class A: 2^24-2 = 16,777,214**
**Class B: 2^16-2 = 65,534**
**Class C: 2^8-2 = 254**

### Deriving the Network ID and Related Numbers
■ Network number
■ First (numerically lowest) usable address
■ Last (numerically highest) usable address
■ Network broadcast address




### Unusual Network IDs and Network Broadcast Addresses
For Class A, the first odd fact is that the range of values in the first octet omits the numbers 0 and 127.

As it turns out, what would be Class A network 0.0.0.0 was originally reserved for some broadcasting requirements, so all addresses that begin with 0 in the first octet are reserved.

What would be Class A network 127.0.0.0 is still reserved because of a special address used in software testing, called the loopback address (127.0.0.1).